Steel sheets and other rolled steels are generally manufactured as Al-killed steels prepared by deoxidizing liquid steels, melted in basic oxygen furnaces, with Al. Alumina formed during deoxidation is hard, tends to form clusters and remains in liquid steel as inclusions of not smaller than several hundred μm.
If such inclusions are not adequately removed from liquid steels, they cause slivers in steel sheets, quality inferiority of structural steel plates, a decrease in low-temperature toughness of wear-resisting steel plates, weld defects in oil-well steel tubes detected by UST (ultrasonic testing) and other defects. Alumina also adheres to and builds up on the inner wall of immersion nozzles during continuous casting and causes nozzle clogging.
Alumina has conventionally been removed from liquid steels by (1) adding Al as a deoxidizer when liquid steel is tapped from the converter so that as much time as possible can be given to the agglomeration, coalescence and floating and separation of alumina from liquid steel after deoxidation, (2) accelerating the flotation and separation of alumina by vigorously stirring liquid steel by CAS (composition adjustment by sealed argon bubbling) or RH (Rheinstahl Huttenwerke und Heraus; vacuum degassing) secondary refining processes, or (3) reforming and rendering innocuous alumina to low-melting inclusion CaO—Al2O3 by adding Ca to liquid steel.
However, floating and separating alumina by said methods (1) and (2) involve a problem that the methods cannot completely remove inclusions not smaller than several hundred μm and prevent slivers on the surface of steel sheets.
Reforming inclusions by said method (3) is capable of preventing the formation of clusters and refining inclusions lowering the melting point thereof.
In order to modify alumina in liquid steel to liquid Ca-aluminate, however, Shirota et al. (refer to Materials and Processes, 4 (1991), p. 1214) say that it is necessary to control the [Ca]/[T.O.] ratio to within the range between 0.7 and 1.2.
In order to conform to this requirement, it is necessary to add, when, for example, T.O. (total oxygen, which is the sum of dissolved oxygen and oxygen in inclusions) is 40 ppm, as much as 28 to 48 ppm Ca to liquid steel.
In steel cords for tires and valve springs, meanwhile, it is generally known to modify and render innocuous inclusions to low-melting CaO—SiO2—Al2O3 (—MnO) type inclusions that are apt to deform during rolling and working.
Still, said method (3) has not been put into practical use in the manufacture of cold-rolled steel sheets for automobiles and cans whose upper limit of Si-content is strictly controlled as Ca is added in the form of low-cost Ca—Si alloys.
There are some known liquid steel deoxidizing methods that use Ce, La or another REM (rare-earth metal). (1) One method based on Al-killing uses REM as alumina modifier after Al-deoxidation and (2) another method uses REM as deoxidizer either singly or in combination with Ca, Mg, etc., without using Al.
As a method based on Al-killing, Japanese Unexamined Patent Publication (Kokai) No. 52-70918 discloses a method for manufacturing clean steel containing few nonmetallic inclusions that removes alumina clusters from liquid steel by causing them to float and separate by controlling the interfacial tension between liquid steel and alumina clusters by adding one or more of Se, Sb, La and Ce of 0.001 to 0.05% after deoxidation with Al or Al—Si, sometimes in combination with stirring of liquid steel.
Japanese Unexamined Patent Publication (Kokai) No. 2001-26842 discloses cold-rolled steel sheets having excellent surface and internal properties and manufacturing method therefore that controls the size of oxide inclusions to 50 μm or under and the composition of said inclusions to Al-oxide of 10 to 30 wt %, Ca-oxide and/or REM of 5 to 30 wt %, and Ti-oxide of 50 to 90 wt %, by adding Ca and/or REM after deoxidizing liquid steel with Al and Ti.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 11-323426 discloses a method for manufacturing clean Al-killed steel with no alumina clusters and few defects by applying composite deoxidation with Al, REM and Zr.
However, these methods have been unable to decrease inclusions defects to desired quality levels because it has been difficult to surely float and separate alumina clusters.
Japanese Patent No. 1150222-discloses a method for manufacturing steel that lowers the melting point of inclusions, and softens the inclusions, by adding an alloy containing one or more of Ca, Mn and REM, for example, of 100 to 200 ppm, after deoxidizing liquid steel with a flux containing Ca-oxide.
Japanese Patent No. 1266834 discloses a method for manufacturing steel wire rods with excellent fine drawability that adds REM of 50 to 500 ppm after controlling T.O. (total oxygen) to 100 ppm or under with a deoxidizer such as Mn or Si, other than Al, with a view to prevent oxidation by air.
However, these methods involve the problem of a cost increase because they do not use low-priced Al as deoxidizer. Deoxidation with Si, according to these methods, is difficult to apply to liquid steel for sheet steels whose upper limit of Si-content is strictly controlled.
Meanwhile, several formation mechanisms have been proposed regarding clustering of alumina particles.
For example, Japanese Unexamined Patent Publication (Kokai) No. 9-192799 discloses that adhesion of Al2O3— particles to immersion nozzles can be prevented by lowering the bonding force of P2O5, which is as binder of Al2O3, by forming nCaO.mP2O5 by adding Ca to liquid steel, based on the knowledge that P2O5 in liquid steel encourages the agglomeration and coalescence of Al2O3.
Yasunaka et al. (Tetsu to Hagane [Iron and Steel], (1995), p. 17) conjecture that alumina particles captured by Ar gas bubbles, which are used for prevention of immersion nozzle clogging in continuous casting, causes slivers in cold-rolled steel sheets.
H. Yin et al. (ISIJ Int., 37 (1997), p. 936 discloses the observation that alumina particles captured by gas bubbles agglomerate and coalesce due to a capillary effect at the surface thereof.
While the forming mechanism of alumina clusters are being elucidated, no concrete methods to prevent clustering have vet been found. It has therefore been difficult to decrease inclusion defects to desired quality levels.